More detail in error messages

[erkyrath]

E.g. "Properties may only be set in action code" (which?)

[erkyrath]

Be even nicer if we tracked in which code property the exception occurs. This probably requires the same work as for "lexical scope".

[ChristopherS]

Maybe it would be possible to add line numbers and print out the line where the error occured? My problem was that I knew the problem was in the desc property, but it has more the 100 lines. So it wasn't easy to track down the error.

[erkyrath]

Yeah, line numbers should be possible too.

For now I have a minimal improvement where it names the property that it failed to assign to.